Cathode ray tube



Sept. 12, 1939.

N. LEVIN GATHODE RAY TUBE Filed Jan. 50, 1937 INVENTOR NYMAN vm BY7fg ATTORNEY Patented Sept. 12, 1939 UNITED STATES PATENT OFFICE GATHOD El RAY TUBE ration of Delaware Application January 30, 1937, Serial No. 123,090 In Great Britain February 3, 1936 6 Claims.

This invention relates to cathode ray tubes and has for its object to provide improved cathode ray tubes wherein relatively large electron beam currents are obtainable.

The invention though not limited exclusively to its application thereto is of particular advantage when applied to television or like cathode ray reproducer tubes. For obvious reasons it is desired in television and like cathode ray re producer tubes to obtain as bright a picture on the fluorescent screen as possible and accordingly it is desirable to increase as much as possible both the electron beam current of the tube and the velocity at which the electrons in the beam impinge on the fluorescent screen but in cathode ray tubes as at present in common use, practical limitations both as regards beam current and final electron velocity are soon reached.

It is Well known that if electrons be projected at suitable velocity upon a suitably prepared electrode, secondary emission will take place therefrom and under suitable conditions the secondary electrons admitted may be made as much as three or four times as great in number as the incident or primary electrons. The present invention utilizes this well known phenomenon of secondary emission to improve cathode ray tubes.

According to this invention a cathode ray tube is provided with at least one apertured electrode and preferably with a plurality of apertured electrodes having the aperture walls adapted to provide secondary emission and means are provided for causing electrons entering the said apertures to bombard the walls thereof and thereby produce 5 secondary emission. In this way amplification of the original primary cathode beam from the cathode is obtained.

The invention is illustrated in the accompanying drawing which shows diagrammatically one 40 embodiment thereof.

Referring to the drawing a cathode ray tube of the television reproducer type comprises within an evacuated envelope a cathode I the electron emission from which is controlled, for example,

45 in accordance with received television signals by means of a suitable apertured control electrode 2 placed close in front of the said cathode. On the side of the control electrode 2 remote from the cathode I is a first anode 3 which has an aper- 50 ture in line with the aperture in the control electrode. The wall 3a of the said aperture may be cylindrical or approximately cylindrical, The disposition of the three electrodes l 2, 3, and the potentials applied thereto are such that electrons 55 from the cathode are so focused as to move as (Cl. 250-160) I indicated by the chain lines in paths which cross over inside the aperture in the anode 3 so that they impinge on the internal surface 3a of this aperture. The said internal surface to is coated with a material of low work functionfo-r eX- 5..

ample caesium-so that it will emit large numbers of secondary electrons. Electrons leaving the first anode pass to a second, similar, anode 4 also with a caesium or similarly internally coated aperture and electrons leaving the said anode 4 10 then pass to a third similar anode 5 also with a caesium or similar internally coated aperture and so on for as many further apertured anodes as may be required. The walls 40,, 5a and so on, of the apertures may be cylindrical or ap- 15 proximately cylindrical like the Wall to. Suitable electron lens arrangements, which may be electromagnetic and/or electro-static in action (in the figure magnetic coils 6, l, are shown) are provided between the successive apertured anodes 20 so that in each case the rays of the beam cross over in such manner that the interior surface of the anode aperture is bombarded by electrons. Each anode may conveniently be maintained at a potential of some 300 to 400 volts positive with 25 respect to the preceding anode in this succession and with suitable design it is possible to obtain at each anode a ratio of secondary emission to incident electrons as high as five.

The actual dimensions to be adopted for the 30 anode electrodes will depend upon design requirements and mainly upon the size of the image formed at the focal points of the cathode ray beam and on the value of the angle between the rays forming the images, i, e., on the value of the angle between the rays in the cones of rays leading to the said focal points. For example if each image is mm. in diameter and the angle of each cone is 30 then, generally, the envelope of the electron beam at the focus will be a c-ylinder about mm. long and about mm. in diameter. If in such a case the cylindrical aperture in each anode be made mm. in diameter and 1 mm. long, practically the whole of the electron beam entering that aperture will be incident upon the wall thereof at some point along its length.

It is preferable in carrying out the invention to make the anode apertures not strictly cylindrical but rather, as shown in the figure, cylindro-conical, each aperture being preferably cylindrical for about tWO- thirds of its length (starting from the cathode end) the aperture then increasing its diameter in conical fashion. This shape of aperture allows of greater field penetration and hence permits secondary electrons to pass more freely. In practice such an aperture may conveniently be made by constituting each anode by a plate bored to form a cylindrical aperture therein and then countersinking the boring on the side remote from the cathode.

A small proportion of primary electrons will be able to pass through each aperture but with a preferred arrangement as above described, such primary electrons would be stopped at the next succeeding anode. This would occur due to the fact that the electrostatic and/or electromagnetic lens arrangements employed will focus differently for the different electron velocities, and therefore, if each lens arrangement is arranged to focus the slower secondary electrons on the aperture of the next succeeding anode whatever proportion of the incident beam which succeeds in passing the aperture in any anode will be stopped by the next anode. The apertured anodes employed in carrying out this invention may conveniently be made from silver sheet, the process of manufacture including cleaning the sheet and baking in vacuo.

Having described my invention, what I claim 1. A cathode ray tube comprising a cathode, a control electrode adjacent to and in register with the cathode, a collector electrode in register with and removed from the cathode, and a plurality of cylindro-conically apertured electrodes, the cylindrical portion only of said apertures being secondary electron emissive in register with the cathode and positioned intermediate the cathode and collector electrode.

2. An electronic device comprising a cathode, a control electrode adjacent to and in register with the cathode, a collector electrode in register with and removed from the cathode, a plurality of cylindro-conically apertured secondary electron emissive electrodes in register with the cathode and positioned intermediate the cathode and collector electrode, and combined electrostatic and electromagnetic means positioned between the apertured electrodes for bringing electrons into focus in the apertures of the secondary electron emissive electrodes.

3. A cathode ray tube including a cathode, an apertured control electrode adjacent thereto for forming electrons from the cathode into a beam and at least one further electrode with a cylindro-conical aperture, the internal wall of the cylindrical portion only being adapted to emit secondary electrons when subjected to electron bombardment, said apertured further electrode being interposed in the path of the formed beam from the control electrode, and means for causing the electrons of said beam to come to a focus in the aperture in said further electrode whereby electrons in said beam bombard the internal wall of said aperture to cause secondary electron emission therefrom.

4. A tube as claimed in claim 3 wherein there is provided in addition a plurality of electrodes each with an aperture having an internal cylindro-conical wall, the cylindrical portion only thereof being adapted to give secondary emission, said apertures being arranged in register with each other along the path of the cathode ray beam, and further means to focus the beam of electrons in the region of the apertures of the electrodes.

5. A tube as claimed in claim 3 wherein the conical portion of the aperture has an increasing diameter in the direction of the path of electrons from the cathode.

6. A cathode ray tube as claimed in claim 3 wherein the cylindrical portion of the aperture is of the order of two-thirds of the total length of said aperture.

NYMAN LEVIN. 

